class Solution9 {

    int m, n;
    int[] dx = {0,0,1,-1};
    int[] dy = {1,-1,0,0};

    public void solve(char[][] board) {
        m = board.length; n = board[0].length;

        for(int i = 0; i < m; i++){
            if(board[i][0] == 'O') { dfs(board, i, 0);}
            if(board[i][n - 1] == 'O') { dfs(board, i, n - 1);}
        }
        for(int j = 0; j < n; j++){
            if(board[0][j] == 'O') { dfs(board, 0, j);}
            if(board[m - 1][j] == 'O') { dfs(board, m - 1, j);}
        }

        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                if(board[i][j] == 'O') { board[i][j] = 'X';}
                if(board[i][j] == '.') { board[i][j] = 'O';}
            }
        }
    }

    public void dfs(char[][] board, int i, int j){
        board[i][j] = '.';

        for(int k = 0; k < 4; k++){
            int x = i + dx[k], y = j + dy[k];
            if(x >= 0 && x < m && y >= 0 && y < n && board[x][y] == 'O'){
                dfs(board, x, y);
            }
        }
    }
}